Light-emitting devices are used in many applications including optical communication systems. Use of optical communication systems continues to increase due to the large signal bandwidth such systems provide. Many optical communication systems operate at relatively long wavelengths, in the range from about 1.2 micrometers (μm) to about 1.6 μm, because optical fibers typically have their lowest attenuation in this wavelength range. Long-wavelength optical communication systems require light sources that emit light at such relatively-long wavelengths. Many optical communication systems use a vertical-cavity surface-emitting laser (VCSEL) as the light source, although other types of light sources are also available.
VCSELs are typically characterized by an optical cavity defined by a pair of mirrors, generally referred to as distributed Bragg reflectors (DBRs), disposed parallel to the major surface of a substrate. The optical cavity generally includes an active region composed of a quantum well structure sandwiched between a pair of spacing layers. The quantum well structure defines one or more quantum wells and is composed of N quantum well layers interleaved with N+1 barrier layers, where N≧1. Each quantum well layer sandwiched between two barrier layers defines a respective quantum well. Typically, no dopants are added to the semiconductor materials of the quantum well structure.
One of the spacing layers is a layer of n-type semiconductor material and the other of the spacing layers is a layer p-type semiconductor material. The n-type and p-type spacing layers inject electrons and holes, respectively, into the quantum well structure and additionally define the distance between the DBRs so that the optical cavity has a thickness equal to an integral multiple of the desired emission wavelength in the materials of the active region.
The DBRs are fabricated of alternating layers of materials that have different refractive indices. The number of layer pairs and the refractive index difference between the materials of the layers constituting the layer pairs collectively define the reflectivity of the DBR. The reflectivity is wavelength dependent and has a peak at the desired emission wavelength. The wavelength at which the reflectivity has a maximum is determined by the thickness of the layers constituting the layer pairs and refractive indices of the materials of the layers. One of the DBRs has fewer layer pairs than the other and is therefore slightly less reflective. The light generated by the VCSEL is emitted through the less-reflective of the DBRs.
Gallium arsenide (GaAs) is by far the most commonly-used substrate material for making VCSELs. Compared to alternatives such as indium phosphide (InP), GaAs wafers are substantially lower in cost, are commercially available in larger diameters and are commercially available with more different orientations of their crystalline surfaces. Also, pairs of semiconductor materials exist that can be epitaxially grown on GaAs with good crystal quality and that have a relatively large difference in their refractive indices. An example of such a pair of materials is GaAs and AlGaAs. DBRs made of such semiconductor materials grown on GaAs require fewer layer pairs to obtain a given reflectivity than DBRs made of semiconductor materials that can be grown on other substrate materials.
In operation, carriers, i.e., electrons and holes, injected into the quantum well structure by the adjacent spacing layers are trapped in the quantum wells, where they recombine to generate light at the desired emission wavelength. The materials of the layers of the quantum well structure are chosen to provide emission at the desired wavelength.
Many conventional types of VCSEL employ semiconductor DBRs in which semiconductor materials are used as the materials of the layer pairs. In an example of such VCSELs, a substrate-side DBR is located on the substrate, the n-type spacing layer, the quantum well structure and the p-type spacing layer are located, in order, on the substrate-side DBR and a remote-side DBR is located on the p-type spacing layer. In such DBRs, current typically passes from an electrode located on the top of the remote-side DBR to an electrode located on the bottom of the substrate. The DBRs are doped to have the same conductivity type as the adjacent spacing layer to make them electrically conductive, i.e., the substrate-side DBR is doped n-type and the remote-side DBR is doped p-type.
Conventional VCSELs have problems with high optical losses and excess Joule heating in their p-type DBRs. These problems can be overcome by incorporating a tunnel junction in the optical cavity, which allows both DBRs to be doped n-type. A VCSEL having two n-type DBRs has lower optical losses and a higher electrical conductivity than one having one n-type DBR and one p-type DBR. A significant benefit of the reduced optical losses is a lower threshold gain and a correspondingly higher differential gain. Differential gain is a key parameter for achieving high-bandwidth modulation in semiconductor lasers, especially in VCSELs.
FIG. 1 shows a side view of an example of a semiconductor device 10 incorporating a tunnel junction structure 12 that defines a tunnel junction. Semiconductor device 10 may be part of a VCSEL, for example. The tunnel junction structure is composed of an n-type tunnel junction layer 14, a p-type tunnel junction layer 16 and a tunnel junction 20 between the tunnel junction layers. The n-type tunnel junction layer is a layer of n-type semiconductor material. The p-type tunnel junction layer is a layer of p-type semiconductor material.
Semiconductor device 10 additionally includes n-type layer 22 on which n-type tunnel junction layer 14 is grown and p-type layer 24 grown on p-type tunnel junction layer 16. N-type layer 22 may constitute the substrate of semiconductor device 10. Alternatively, n-type layer 22 may be grown on or over the substrate.
Applying a reverse bias across tunnel junction 20 causes a tunneling current to flow across the tunnel junction. A reverse bias is applied by setting n-type tunnel junction layer 14 to a more positive voltage than p-type tunnel junction layer 16. The reverse conduction characteristic of tunnel junction 20 has a threshold characteristic, i.e., a voltage greater than a threshold voltage has to be applied across the tunnel junction before a significant tunneling current will flow. It is desirable to minimize the voltage drop across tunnel junction 20 to reduce the overall voltage drop across semiconductor device 10 and to reduce power dissipation in the tunnel junction.
The voltage drop across tunnel junction 20 can be reduced by forming tunnel junction structure 12 of semiconductor materials that establish a large built-in electrostatic field across the tunnel junction. A large electrostatic field requires a large potential difference across a short distance. Conventional approaches to minimizing the voltage drop across the tunnel junction have focused on minimizing the width of the depletion region at the tunnel junction by maximizing the doping concentrations in the semiconductor materials of tunnel junction layers 14 and 16.
FIGS. 2A and 2B each include a schematic energy diagram 40 and an electrical circuit model 42 that show some of the characteristics of tunnel junction structure 12. FIG. 2A shows the characteristics of the tunnel junction structure with no bias applied. FIG. 2B shows the characteristics of the tunnel junction structure under reverse bias. Each energy diagram shows the conduction band energy ECN and the valence band energy EVN of the semiconductor material of n-type tunnel junction layer 14 and additionally show and the valence band energy EVP of the semiconductor material of p-type tunnel junction layer 16. N-type tunnel junction layer 14 and p-type tunnel junction layer 16 collectively form tunnel junction 20. Each energy diagram also shows the Fermi level EFN of the semiconductor material of n-type tunnel junction layer 14 and the Fermi level EFP of the semiconductor material of p-type tunnel junction layer 16.
The energy diagram of FIG. 2A shows the depletion region 44 that exists at tunnel junction 20 with no bias applied. With no bias applied, the Fermi level EFN of the semiconductor material of n-type tunnel junction layer is equal to the Fermi level EFN of the semiconductor material of p-type tunnel junction layer 16. The conduction bands of the materials of the tunnel junction layers differ in energy, which establishes the built-in potential barrier 46 at the tunnel junction that prevents conduction through the tunnel junction at low forward bias. The electrostatic field strength at the tunnel junction depends on the height of the built-in potential barrier and depends inversely on the width W of depletion region 44.
A forward bias applied across tunnel junction 20 decreases the height of the built-in potential barrier at the tunnel junction. Sufficient forward bias causes current to flow across the tunnel junction in the forward direction. A forward bias is established by setting p-type tunnel junction layer 16 to a more positive voltage than n-type tunnel junction layer 14. The width of depletion region 44 decreases under forward bias (not shown).
A reverse bias applied across tunnel junction 20 as shown in FIG. 2B adds to the height of the built-in potential barrier and increases the width of depletion region 44 to W′. The reverse bias separates the Fermi levels EFN and EFP on opposite sides of the tunnel junction. In the example shown, the Fermi level EFP of the material of p-type tunnel junction layer 16 has increased relative to its no-bias level, whereas the Fermi level EFN of the material of n-type tunnel junction layer 14 remains substantially unchanged. In a conventional p-n junction, the large width of the depletion region allows only a small leakage current to flow across the junction under reverse bias. However, in tunnel junction 20, the reverse bias applied across the narrow depletion region causes electrons tunnel through the potential barrier from the valance band of p-type tunnel junction layer 16 to the conduction band of n-type tunnel junction layer.
The reverse bias elevates the valence band energy EVP of the semiconductor material of p-type tunnel junction layer 16 to a level above the conduction band energy ECN of the semiconductor material of n-type tunnel junction layer 14. This allows electrons in the valence band of the semiconductor material of the p-type tunnel junction layer to tunnel through the potential barrier to unoccupied sites in the conduction band of the semiconductor material of n-type tunnel junction layer 14, as shown schematically in FIG. 2B. The greater the reverse bias applied across tunnel junction 20, the higher the probability that an electron, e−, will tunnel across tunnel junction 20, and the higher the conduction across the tunnel junction.
In conventional tunnel junction structure 12, the semiconductor materials of tunnel junction layers 14 and 16 have a relatively large band-gap energy difference. Such materials establish a relatively high tunneling barrier at tunnel junction 20. A large reverse bias therefore has to be applied across the tunnel junction structure to cause a tunneling current of the order of the laser current of a laser diode to flow. Such tunnel junction structures therefore typically have a high voltage drop, which is usually undesirable. As noted above, the voltage drop across the tunnel junction is conventionally reduced by doping the tunnel junction layers at as high a dopant concentration as possible. This reduces the width of the depletion region and thereby increases the tunneling probability. If even one of the tunnel junction layers is not doped to a doping concentration that provides degeneracy, an additional reverse bias equal to the voltage difference between the Fermi level and the band edge must be applied before any tunneling commences.
Thus, what is needed is a tunnel junction structure for use in GaAs-based lasers that will have a lower voltage drop than conventional GaAs-based tunnel junction structures.